Various types of color display technologies are known in the art. For example, there are CRT display systems, LCD systems, and projection display systems. In front projection displays, the projected images are viewed from a reflective viewing screen. In rear projection displays, the projected images are viewed through a transmissive viewing screen.
To produce color images, existing display devices use three primary colors, typically, red green and blue, collectively referred to as RGB. In sequential projection display systems, the three primary color components of an image are modulated and displayed sequentially, typically using a single Spatial Light Modulator (SLM) panel. In simultaneous projection display systems, the three primary color components of the image are modulated and displayed simultaneously using one or more SLM panels.
An important consideration in designing projection display devices is the display brightness. Thus, efforts are continually made to increase the optical efficiency of existing designs and, thereby, to increase the luminous output that can be obtained from a given light source.
Unfortunately, the light sources commonly used in existing display devices, for example, the UHP™ lamps available from Philips Lighting, a division of Royal Philips Electronics, Eindhoven, Netherlands, produce non-uniform light spectra wherein, typically, the intensity of the red wavelength range is significantly lower than the intensity of other spectral ranges. Thus, in existing RGB systems, typically, higher brightness may be achieved only by significantly reducing the color saturation of the red wavelength ranges. Further, in projection display systems for home theater applications, wherein highly saturated colors are typically required, filters with narrower spectral transmission ranges are typically used, causing an additional reduction in image brightness.
The quality of color image reproduction can be significantly improved by expanding the color gamut of the display system. This can be achieved by using more than three primary colors to reproduce the image. Display systems using more than three primary colors are described in International Application PCT/IL01/00527, entitled “Device, System and Method For Electronic True Color Display”, filed Jun. 7, 2001, and published Dec. 13, 2001 as WO 01/95544, assigned to the assignee of the present application, the entire disclosure of which is incorporated herein by reference.
A six-primary display using superimposed images produced by two projection display devices, wherein each projection display device uses three different primary colors, is described in Masahiro Yamaguchi, Taishi Teraji, Kenro Ohsawa, Toshio Uchiyama, Hideto Motomura Yuri Murakami, and Nagaaki Ohyama, “Color image reproduction based on the multispectral and multiprimary imaging: Experimental evaluation”, Device Independent Color, Color Hardcopy and Applications VII, Proc. SPIE, Vol. 4663, pp. 15-26 (2002). In the dual-projection display system described in this reference, the wavelength ranges selected for the six primary color filters are essentially uniformly distributed across the visible spectra of 400-700 nm, with no spectral overlap between the primaries. In this way, a wide gamut may be achieved; however, the combined brightness of the two projection devices is dramatically reduced. In fact, the combined brightness produced by this dual-projection device is lower than the brightness produced by a corresponding single RGB projection device. Dividing the visible spectrum into six (rather than three) ranges does not increase the over-all image brightness because the six primaries cover narrower sub-ranges of the same visible spectrum. An additional reduction of intensity is caused by inherent optical losses in the division of the spectrum into narrower ranges.